GEC Dioptrion 250w/400w MBF lantern. Introduced by GEC in 1948, the Dioptrion was a popular mercury lantern whose design was quite revolutionary in that the canopy was partly constructed from prismatic glass. The all-round combination of glass bowl and canopy is unusual, and it can only be assumed that the combined optics of the prismatic canopy and bowl distributed the majority of lamplight towards the road surface below. However, a significant amount of light must have escaped above the horizontal, as there is no reflector plate in the canopy to prevent this from occurring.

There are two Dioptrions held in the collection; the first example acquired came from the City of Worcester where it was situated in Pump Street until its removal in 2005, having given some 50-years continuous service. The City of Worcester still has three or four of these lanterns in use in heritage areas of the City (believed to have gone by April 2006). The second example came from an old Industrial Estate in Staffordshire in February 2006 and had been long disused. However, it is complete and totally original.

With the advent of commercially available 'tubed' discharge lamps, manufacturers quickly realised that significantly improved light distribution could be obtained if the lamps were burning in a horizontal attitude, rather that the vertical; the Dioptrion is an early example of a commercial lantern designed for use with horizontally mounted mercury lamps. However, these medium pressure lamps (MA lamps) couldn't effectively burn in a horizontal orientation as the convection currents in the discharge tube within the lamp caused the arc to bow upwards, eventually destroying the lamp's aluminosilicate hard glass arc tube. To overcome this problem, lanterns such as the Dioptrion were fitted with electro magnetic arc-defectors to counteract the 'bowing' effect on the arc. However, with the advent of improved lamp technology and the introduction of the quartz glass arc tube, the high-pressure mercury lamp (MB lamp) was born, and these could burn in any desired attitude. These new MB lamps were made commercially availably in the 1950's and were known as MB/U lamps (the 'U' denoting a 'universal' burning attitude); therefore arc-deflectors were no longer necessary where MB/U lamps were used. The example that originated from Worcester has lost its electro-magnetic arc-deflector sometime in the distant past; however, the example acquired from Staffordshire still retains its arc-deflector despite a MB/U 400w lamp being fitted (see photo).

The Dioptrion was once popular with many local authorities throughout the UK, and it was still common to find examples surviving into the 1970's. Today, very few exist and those that do are often damaged or vandalised; not surprisingly, it has taken me many years to locate and acquire the two examples now held in the collection.  

Both Dioptrions are currently stored and await restoration.

If you want more detailed technical information on discharge lamps, then do visit James Hooker's excellent website at www.lamptech.co.uk.

This lovely example came from the City of Worcester where it had remained in use until 2005. Because it had been in constant use until its removal in 2005, it remains in excellent condition. It is seen in acquired condition in August 2005.  

The GEC Dioptrion was suitable for both top-entry and side-entry mounting. The side-entry mounting hole on this lantern is blanked off.

Access to the Dioptrion's interior is made by releasing the two end toggle fasteners, which allows the bowl of the lantern to hinge downwards on a forward mounted hinge. The Dioptrion is a superbly engineered lantern, and all toggle fasteners and hinges worked perfectly on both examples, despite many decades of being exposed to the elements.

A sectional drawing of the Dioptrion from GEC's 1948 catalogue. These lanterns are 10 ¾ " in overall height and 22 ¾ " in overall length. A sectional view of the electro magnetic arc-deflector can be seen above the lamp.

The 'Staffordshire' Dioptrion showing the Arc Deflector mounted above the lamp. Care needs to be taken with the crumbly asbestos present in these old lanterns.

The disused 'Staffordshire' GEC Dioptrion when still in situ in September 2005. Although originally intended for MA/H lamps, this had a 400w MB/U lamp fitted.

The same Dioptrion as seen above with part of the concrete bracket still attached!  Sadly the glass canopy has been cracked at some point in the distant past. The Biro pen gives some idea as to the size of the Dioptrion lantern.

A GEC Dioptrion on a highly decorative cast iron bracket and column in Worcester. The Dioptrion is most likely a second generation lantern for this lovely old column. Note the two-part photocell sticking out of the top of the lantern.